Four-channel reproducing system

ABSTRACT

A signal converter is provided for mixing first and second channel signals from a conventional two-channel source to produce first and second composite signals. Phase differences between the first and second composite signals with respect to the first and second channel signals respectively vary as a function of frequency of the first and second channel signals. The first and second channel signals or the first and second composite signals are applied to a second signal converter for producing fourchannel output signals by mixing input signals. The second signal converter is operative to vary mixing ratios of input signals in accordance with the phase relationship between the first and second composite signals.

United States Patent 1191 Ito et al.

1 FOUR-CHANNEL REPRODUCING SYSTEM [75] Inventors: Ryosuke Ito; SusumuTakahashi.

both of Tokyo, Japan [73] Assignee: Sansui Electric Co., Ltd., Tokyo,

Japan [22] Filed: Dec. 26, 1973 [21] Appl. No.: 428,423

Related US. Application Data [63] Continuation-impart of Ser. No.315,928, Dec. 18,

11] 3,889,061 51 June 10, 1975 3,757,047 9/1973 lshida et a1 179/1 GOPrimary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas DAmicoAttorney, Agent, or FirmHarris, Kern, Wallen &

Tinsley [57] ABSTRACT A signal converter is provided for mixing firstand second channel signals from a conventional two-channel source toproduce first and second composite signals. Phase differences betweenthe first and second composite signals with respect to the first andsecond 52 n 1 I channel signals respectively vary as a function of fre-1 U S Cl quency of the first and second channel signals. The [51] Int Clfirst and second channel signals or the first and second I I i o t I l Is s o u u r a [58] Fleld Ofsearch -179/1GQ11OO'4 verter for producingfour-channel output signals by 179/100'1TD-15 BT mixing input signals.The second signal converter is [56] Ref Ci operative to vary mixingratios of input signals in ac- UNITED STATES PATENTS cordance with thephase relationship between the first 3,170,991 2/1965 Glasgal 179/1 GOand Second composlte Slgnals' 3.691692 10 1972 Hafler 179/1 60 3 Claims,17 Drawing Figures 3,745,254 7/1973 Ohta 179/1 GQ t 1:5 FL r L C E A 009 17 5 5 FR a o 2 Lu 8 :2 RL 1 E i o-eo 2 CC 1- o RR 21 RC (7) E x (IECi CONTROL UNIT 5C2 1% 6890651 JUN 10 I975 I PATENTED SHEET 4PATENTEDJUN 1 0 I915 a 8 89,061

SHEET 6 F I G. 10

\(ECZ l I l i F l G. H

H 122 MATRIX J-'- SIGNAL 2-CHANNEL 4-CHANNEL SOURCE R CONVERTER (R,REPRODUCER SHEET fill-R) Er E2 R, I-DISCRIMINATOR TOR iTi

DISCRIMINA LI FR PHASE PATENTEDJUH I 0 I975 ECi - CONTROL F l G. 13

PATENTED SHEET FIG. 15

FREQUENCY FIG. l6

FIG. 17

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FREQUENCY F OUR-CHANNEL REPRODUCING SYSTEM CROSS-REFERENCE TO RELATEDAPPLICATION This is a Continuation-impart of U.S. Patent applicationSer. No. 315,928, filed Dec. 18, 1972.

This invention relates to a signal converting system which is utilizedto reproduce two-channel signals from a conventional two-channel sourceby a four-channel reproduction system and capable of enhancing theseparation between signals in a reproduced sound field.

Recently, a matrix four-channel sound reproduction system has been usedwherein four-channel original signals are converted into two-channelsignals, the twochannel signals are recorded on such recording medium asa phonograph record or a magnetic tape, the two-channel signalsreproduced from the recording medium are converted into four-channelsignals corresponding to the original signals and the four-channelsignals are reproduced by four loudspeakers arranged around a listener.

The matrix four-channel reproducing system, however, involves a seriousproblem that the crosstalk be tween reproducing channels is extremelylarge. Specifically, in one type of the matrix four-channel reproducingsystems the separation between channels disposed in a diagonal directionis infinity whereas that between adjacent channels equals 3 db.

Although the matrix four-channel reproducing system has beensuccessfully developed as above pointed out, the number of matrixfour-channel stereo records now on the market is far smaller than thatof twochannel stereo records. The matrix four-channel reproducing systemis compatible with conventional twochannel stereo records so that it ispossible to enjoy a four-channel playback of a conventional two-channelstereo record.

However, owing to the inherently poor separation characteristic of thematrix four-channel reproducing system, when reproducing a conventionaltwo-channel stereo record by a four-channel system, the rear sidelocation of the listening area of a sound image presents a problem. Moreparticularly, where a two-channel stereo record is reproduced by afour-channel system, when only a left signal is reproduced from therecord it is desirable to locate the sound image based on this signal atthe rear-left side of the listening room for the purpose of providing asatisfactory four-channel reproduction. However, with the systemdescribed above, the sound image will be located at an intermediatepoint between the front-left side and the rear-left side. This means apoor separation between the front and rear channels.

It is an object of this invention to provide an improved signalconverting system capable of enhancing the separation between thechannels when reproducing sound signals from a conventional two-channelstereo recording medium by a four-channel reproduction system.

According to another aspect of this invention there is provided a signalconverting system for reproducing two channel stereo signals from a twochannel signal source by means of a four-channel stereo reproducingsystem, said system comprising a combination of: first signal convertingmeans connected to receive the first and second channel signals fromsaid two channel signal source for combining the first and secondchannel signals at a relative amplitude ratio and in a phaserelationship to produce first and second composite signals; a controlunit responsive to the phase relationship between the first and secondcomposite signals for producing first and second control outputs; andsecond signal converting means connected to receive the two channelsignals for producing four-channel output signals, said second signalconverting means including means for combining the two channel signalsat relative amplitude ratios and in phase relationships therebetween toproduce the four-channel output signals and means responsive to thefirst and second control outputs from said control unit for controllingat least relative amplitude ratios between the two channel signalscontained in the four-channel output signals.

According to still another aspect of this invention there is provided asignal converting system for reproducing two channel stereo signals froma two channel signal source by means of a four-channel stereo reproducing system comprising a combination of: first signal converting meansconnected to receive the first and second channel signals from said twochannel signal source for combining the first and second channel signalsat a relative amplitude ratio and in a phase relationship to producefirst and second composite signals; a control unit responsive to thephase relationship between the first and second composite signals forproducing first and second control outputs; and second signal convertingmeans connected to receive the first and second composite signals forproducing four-channel output signals, said second signal convertingmeans including means for combining the first and second compositesignals at relative amplitude ratios and in phase relationshipstherebetween to produce the fourchannel output signals and meansresponsive to the first and second control outputs from said controlunit for controlling at least relative amplitude ratios between thefirst and second composite signals contained in the four-channel outputsignals.

This invention can be more fully understood from the following detaileddescription when taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 shows asblock diagram of a signal converting system embodying theinvention;

FIG. 2 shows a'connection diagram of a matrix circuit shown in FIG. 1;

FIG. 3 shows a block diagram of a portion of a modified embodiment ofthis invention;

FIG. 4 shows a modification of the embodiment shown in FIG. 3;

FIGS. 5, 6 and 7 show connection diagrams of different signal convertersutilized in this invention;

FIG. 8 shows a connection diagram of one example of a control unitutilized in the embodiments shown in FIGS. 3 and 4;

FIG. 9 shows a connection diagram of one example of a variable matrixcircuit utilized in the embodiments shown in FIGS. 3 and 4;

FIG. 10 is a graph showing the output characteristic of the control unitshown in FIG. 8;

FIG. 11 is a simplified block diagram of still another embodiment of theinvention;

FIGS. 12, 13 and 14 show block diagrams of other variable matrixcircuits;

FIG. 15 is a circuit diagram of a signal converter according to anotherembodiment of this invention; and

FIGS. 16 and 17 are characteristic diagrams of the signal convertershown in FIG. 15.

With reference now to FIG. 1 of the accompanying drawing illustrating apreferred embodiment of this invention, reference numeral designates asuitable two-channel source which may be a conventional stereophonograph record, a stereo recorded magnetic tape or an FM stereoreceiver. The first and second audio signals L and R produced by thetwo-channel source 10 are applied to a matrix circuit 12 via a signalconverter 11 to be described later. The matrix circuit 12 may beconstructed as shown in FIG. 2, for example. The circuit shown in FIG. 2is constructed to convert the first and second audio signals L and Rinto fourchannel signals consisting of FL (front-left), FR (frontright),RL (rear-left) and RR (rear-right), which are expressed by the followingequations:

where each A represents a matrix coefficient having a value of about0.4. In the four-channel reproducing system, it is usual to install fourloudspeakers SFL, SFR, SRL and SRR about a listener 13 in a listeningroom 14.

The output FL from matrix circuit 12 is applied to the correspondingloudspeaker SFL through a phase shifter 15 and a power amplifier 16while the output FR to loudspeaker SFR through a phase shifter 17 and apower amplifier 18. Similarly, the outputs RL and RR are supplied tocorresponding loudspeakers SRL and SRR respectively through phaseshifters 19 and 21 and power amplifiers 20 and 22. The purpose of thephase shifters 15, 17, 19 and 21 is to maintain front signals FL and FRat the in-phase relationship throughout the entire range of audiofrequencies and to bring the rear signals RL and RR into in-phaserelationship which have been 180 out-of-phase.

Where only a left signal is impressed upon the matrix circuit 12, bothsignals FL and RL are designated by L. Accordingly, under theseconditions, although it is desirable to locate the sound image of thisleft signal L at the position of the loudspeaker SRL, actually the soundimage is located at a mid-point between the loudspeakers SFL and SRL. Aswill be described later, the signal converter 11 is constructed to formdifference signals L (L-aR) and R (R-aL) in response to the left andright signals. Accordingly, responsive to the signals L-aR and R-aL, thematrix circuit 12 operates to form the following signals:

Accordingly, where only the left signal L is impressed upon signalconverter 11, the outputs FL and RL from the matrix circuit 12 are shownby L(1Aa) and L(1+Aa), respectively. This means that the sound imagecorresponding to the left signal L is located at a position closer tothe loudspeaker SRL.

One example of the signal converter 11 shown in FIG. 5 is provided withinput terminals 21 and 22 connected to receive the left and rightsignals L and R, respectively, and a pair of output terminals 23 and 24.

The first input terminal 21 is connected to the input terminal of afirst inverter 25, and a first potentiometer resistor 26 is connectedbetween the output terminal of the first inverter 25 and the first inputterminal 21. The sliding arm 27 of the first potentiometer resistor 26is connected to the second input terminal 22 through serially connectedresistors 28 and 29, and the junction between these resistors 28 and 29is connected to the second output terminal 24. The second input terminal22 is connected to the input terminal of a second inverter 30, and asecond potentiometer resistor 31 is connected between the outputterminal of the second inverter 30 and the second input terminal 22. Thesliding arm 32 of the second potentiometer resistor 31 is connected tothe first input terminal 21 through serially connected resistors 33 and34, the junction therebetween being connected to the first outputterminal 23. The sliding arms 27 and 32 of the first and secondpotentiometer resistors 26 and 31 are mechanically interlocked as shownby dotted lines.

When the sliding arms 27 and 32 are positioned at the centers of firstand second potentiometers 26 and 31 the left and right signals L and Rare produced at the first and second output terminals 23 and 24,respectively.

When the sliding arms 27 and 32 are moved in the direction of arrows aalong the potentiometer resistors 26 and 31, respectively, there arerespectively derived from the output terminals 23 and 24 two differencesignals L-ozR and R-aL each having varying relative amplitude ratiobetween the signals L and R, whereas when the sliding arms 27 and 32 aremoved in the direction of arrows b there are obtained two sum signalsL+BR and R-l-BL each having varying relative amplitude ratio. In theembodiment shown in FIG. 1 it is advantageous to use the potentiometerresistors 26 and 31 such that their sliding arms are positioned to theright of their mid-points to produce the two difference signals.

In a modified signal converter shown in FIG. 6, each of thecollector-emitter paths of first and second transistors Q and Q isconnected across a source indicated by +B and the ground, and the baseelectrodes of these transistors are connected to input terminals 21 and22 respectively through coupling capacitors. In parallel with thecollector-emitter paths of the transistors Q and Q, are connected firstand second potentiometer resistors 36 and 37 provided with sliding arms38 and 39, respectively. The sliding arm 38 of the first potentiometerresistor 36 is connected to the emitter electrode of transistor Qthrough serially connected resistors 40 and 41, whereas the sliding arm39 of the other potentiometer resistor 37 is connected to the emitterelectrode of transistor Q through serially connected resistors 42 and43. Junctions between resistors 42 and 43 and between 40 and 41 areconnected to output terminals 23 and 24, respectively. The sliding arms38 and 39 of two potentiometer resistors 36 and 37 are mechanicallyinterlocked each other as shown by dotted lines.

In this embodiment, the collector resistors 44 and 45 and the emitterresistors 46 and 47 of transistors Q, and Q are made to have an equalvalue. Again, when sliding arms 38 and 39 are moved in the direction ofdotted arrows a two difference signals L-aR and RaL are produced at theoutput terminals 23 and 24 respectively whereas when these sliding armsare moved in the direction of solid line arrows b two sum signals L-l-BRand R+BL are produced.

In another embodiment of the signal converter shown in FIG. 7, each ofthe collector-emitter paths of transis tors Q and O is connected acrossthe source. The base electrodes of these transistors are connected toinput terminals 21 and 22 respectively through coupling capacitors whilethe collector electrodes are connected to output terminals 23 and 24respectively. The emitter electrodes of transistors Q and Q, areinerconnected through a resistor R. In this signal converter twodifference signals each having a predetermined fixed amplitude ratiobetween the signals L and R are derived out from output terminals 23 and24.

A modified embodiment of this invention shown in FIG. 3 comprises avariable matrix circuit 48 and a control unit 49. This modificationillustrates a decoder capable of reproducing with satisfactory channelseparation sound signals recorded on a matrix four-channel recordingmedium by a four-channel system. In such a decoder the phaserelationship between the twochannel signals, for example, LLF+AFR+jRL+jARR and R FR+AFLjRRjARL which are reproduced from the matrixfour-channel recording medium is detected by the control unit 49constituted by a phase discriminator or a level comparator, and thematrix coefficients of the matrix circuit 48 are controlled by theoutputs ECl and EC2 from the control unit 49. When the two-channelsignals reproduced from a conventional two-channel recording medium areapplied to such a decoder system the control unit 49 can not control thematrix circuit 48 because the two-channel signals are generally inphase. For this reason, in such a case, it is desirable to providesignal converter 11 as shown in FIG. 5, 6 or 7 on the input side of thecontrol unit 49 so that when only one signal is reproduced from thetwo-channel recording medium, the input signals to the control unit 49will have opposite phases thereby enabling the control unit 49 tocontrol the variable matrix circuit 48. For example, where only L signalpresents, it is possible to locate the sound image of the signal at theposition of loudspeaker SRL by the operation of the variable matrixcircuit 48, thus improving the separation between the front channels andthe rear channels.

To aid the understanding of the invention, the constructions andoperations of variable matrix circuit 48 and control unit 49 will bedescribed briefly hereunder.

FIG. 8 shows a circuit diagram of a phase discriminator which comprisesa first limiter 50 including transistors 51 and 52 connected to receivethe L signal and a second limiter 53 including transistors 54 and 55connected to receive the R signal. The first and second limiters 50 and53 have large amplification gains and operate to transform the signals Land R into rectangular wave signals. Two output signals of oppositepolarities produced by the second limiter 53 are amplified by first andsecond amplifiers 56 and 58 including transistors 57 and 59respectively. The outputs from the first and second amplifiers 56 and 58are supplied to a first switching circuit 60 and a second switchingcircuit 61 respectively including bridge connected diodes D, to D anddiodes D to D thereby causing these switching circuits ON and OFFalternately. The output from the first limiter 50 is coupled to thecommon input of the first and second switching circuits 60 and 61, whilethe output terminals of these switching circuits 60 and 61 are groundedthrough capacitors 62 and 63 respectively, and are connected to a pointof reference voltage (in this case, +B/2 volts) through potentiometers64 and 65, respectively. The slidable arms of the potentiometers 64 and65 supply the first and second control outputs ECl and EC2.

The phase discriminator constructed as above described operates toswitch the left signal L by alternately rendering ON and OFF the firstand second switching circuits 60 and 61 in response to the right signalR thereby discriminating the phase difference between the right and leftsignals R and L. FIG. 10 shows the operating characteristic of the phasediscriminator showing that the first and second control outputs ECl andEC2 vary symmetrically but in opposite directions about the referencelevel, which is equal to about +B/2 volts in the phase discriminatorshown in FIG. 8.

FIG. 9 illustrates an example of the variable matrix circuit 48 whereina first matrix circuit associated with the front channels comprises afirst differential amplifier 91 including transistors 92 and 93. Theleft signal L is coupled to the base electrode of transistor 92 whilethe right signal R is coupled to the base electrode of transistor 93through an inverter 94 including a transistor 95. The collectorelectrode of transistor 92 is connected to the first output terminal ofthe matrix circuit while the collector electrode of transistor 93 isconnected to the second output terminal of the matrix circuit through aninverter 96 comprising a transistor 97. A first control circuit 99including a field effect transistor 100 is capacitively connected inparallel with a common emitter resistor 98 of transistors 92 and 93which constitute the differential amplifier 91. The gate electrode ofthe field effect transistor 100 is connected to a control input terminalso that it acts as a variable resistor. The first control circuit 99operates to vary the AC impedance of the emitter circuits of transistors92 and 93 in accordance with the magnitude of the control input ECl soas to control the common mode gain of the differential amplifier 91.

The second matrix circuit associated with the rear channels comprises asecond differential amplifier 106 including transistors 107 and 108. Theleft signal L is coupled to the base electrode of transistor 107,whereas the right signal R is coupled to the base electrode oftransistor 108. The collector electrodes of transistors 107 and 108 arerespectively connected to the third and fourth output terminals of thematrix circuit. A second control circuit including a field effecttransistor 111 is capacitively connected in parallel with a commonemitter resistor 109 for transistors 107 and 108. The gate electrode offield effect transistor 111 is connected to a control input terminal.The second control circuit 110 operates in the same manner as the firstcontrol circuit 99 so as to control the common mode gain of the seconddifferential amplifier in accordance with the magnitude of the controlinput EC2.

The operation of the variable matrix circuit shown in FIG. 9 will bebriefly described as follows: Where the composite signals L and R aresubstantially in phase, the control input ECl is large and the controlinput EC2 is small. Consequently, the AC impedance of the emittercircuits of transistors 92 and 93 is decreased whereby the gain of thefirst differential amplifier 91 is increased, whereas that of the seconddifferential amplifier 106 is decreased. Increase in the gain of thefirst differential amplifier 91 results in the increase in the level ofthe left signal L which is derived out from the collector electrode oftransistor 92 and in the decrease in the level of the right signal Rcontributing to increasing the crosstalk. On the other hand, the levelof the right signal R derived out from the collector electrode oftransistor 93 is increased and the level of the left signal Lcontributing to increasing the cross-talk is decreased. Accordingly, theseparation between the front channels is improved with the increase inthe signal level. In the rear channels, as the gain of the seconddifferential amplifier 106 decreases, the separation degrades with thedecrease in the signal level.

In FIG. 3, when the signal converter 1 1 produces two difference signalsL-aR and R-aL in in-phase relationship, the outputs EC] and EC2 of thecontrol unit 49 have a large level and a small level, respectively.Then, the gains of the first and second differential amplifiers 91 and106 are respectively increased and decreased, and the first output FLand second output FR of the first matrix circuit 90 are formed mostly ofsignal L and signal R, respectively. Thus, signal L is localized at theloudspeaker SFL, and signal R at the loudspeaker SFR.

If supplied with only signal L or signal R, the signal converter 11produces two output signals L and AL or two output signals R and AR bothin reverse-phase relationship. As a result, the outputs ECI and EC2 ofthe control unit 49 have a small level and a large level, respectively,thereby to decrease the gain of the first differential amplifier 91 andto increase the gain of the second differential amplifier 106.Consequently, the outputs RL and RR of the second matrix circuit 105come to be filled mostly with signal L and signal R, respectively.That'is, if only signal L is supplied to the converter 11, it' islocalized at the loudspeaker SRL. Similarly, if only signal R issupplied to the converter 11, it is localized at the loudspeaker SRR.

To generalize the above, signals to be localized somewhere to the leftand to the right of midway between left and right loudspeakers for2-channel reproduction are localized at the loudspeaker SFL and at theloudspeaker SFR, respectively. Signals to be localized at the left andright loudspeakers are localized at the loudspeakers SRL and SRR,respectively.

When the signal converter 11 produces two sum signals L+BR and R+BL, theoutputs ECl and EC2 of the control unit 49 have a large level and asmall level, respectively. As a result, the in-phase components in theinput signals L and R are localized in front of the reproduction soundfield, and the reverse-phase components, e.g. reverberation components,of these signals are localized at the back of the reproduction soundfield.

The detail of the construction and operation of the variable matrixcircuit 48 and the control unit 49 and their modifications are fullydescribed in the copending US. Patent application Ser. No. 298,933,filed Oct. 19, 1972, of the title Decoder for use in 4-2-4 matrixplayback system now US. Pat. No. 3,825,684.

FIG. 4 shows a modification of the circuit shown in FIG. 3.

In this modification, the variable matrix circuit 48 and control unit 49are connected to receive the two channel signals L and R via the signalconverter 11. When the two channel signals L and R are in-phase andinclude crosstalk components therebetween, the difference signalsproduced by the signal converter 11 are caused to be decreased in levelas well as enhanced in separation. Accordingly, the arrangement of FIG.4 in which the separation previously enhanced two-channel signals aresupplied to the variable matrix circuit 48 can provide somewhat betterseparation characteristics than the arrangement of FIG. 3.

FIG. 12 is a block diagram of a variable matrix circuit according toanother embodiment used in the arrangement of FIG. 4. With referencefirst to the front channels, there are provided a first matrix circuitadapted to produce sum signals (L'+R') and (L'+R) of opposite polaritiesfrom the composite signals L and R produced by the signal converter 11,and a second matrix circuit 131 adapted to produce a difference signal(L'R'). The difference signal (L'R') is applied to a third matrixcircuit 133 via a variable gain amplifier 132 to be added therein to theoutputs from the first matrix circuit 130. The first variable gainamplifier 132 is controlled by the first control output ECl from thecontrol unit 49 and has an amplification gain f which varies from 0 to2.41 with respect to the gain of the first matrix circuit 130. The thirdmatrix circuit 133 functions to produce a first output expressed by(l+f)L (1f)R and a second output expressed by (lf)L' (1+f)R which isphase inverted by an inverter 134.

Associated with the rear channels there are provided a fourth matrixcircuit 135 adapted to produce difference signals (L'R') and (L'R) ofthe opposite polarities and a fifth matrix circuit 136 adapted toproduce a sum signal (L'+R'). The sum signal (L'+R') is applied througha second variable gain amplifier 137 to a sixth matrix circuit 138 whereit is added to the outputs (L'R') and (L'R) from the fourth matrixcircuit 135.

The second variable gain amplifier 137 has an amplification gain b whichvaries from O to 2.41 with respect to the gain of the fourth matrixcircuit 135. Accordingly, the sixth matrix circuit 138 produces a thirdoutput expressed by (1+b)L (1b)R' and a fourth output expressed by(1+b)R (1-b)L. The gains of the first and second variable gainamplifiers 132 and 137 are varied in the opposite directions by thecontrol outputs ECl and EC2 from the control unit 49.

FIG. 13 shows a block diagram of another embodiment of the variablematrix circuit. FIG. 13 is different from FIG. 12 in that it isincorporated with the following circuit components. More particularly,there are provided a 0 phase shifter and a 45 phase shifter 171 whichintroduce a phase difference of 45 between the composite signal L andthe composite signal R. Responsive to the outputs from phase shifters170 and 171 an adder 172 provides an output (L'+R +45), whereas asubtractor 173 provides an output (LR +45). A phase discriminator 174for controlling the left and right channels operates to detect the phasedifference between the output signals (L+R +45) and (LR +45) to producecontrol outputs Er and El. A matrix circuit 175 is connected to receivethe signal L and the signal R through a variable gain amplifier 176 toproduce outputs FL3 (=L'+IR) and RL3(=LIR). Similarly, a matrix circuit177 is connected to receive the signal L through a variable gainamplifier 178, and the signal R to produce outputs FR3(=R+rL) andRR3(=R'-RL') where l and r represent the gains of the variable gainamplifiers 176 and 178 respectively. These gains are controlled in theopposite directions in a range of from 0 to 3.414 by the outputs Er andEl from the phase discriminator 174. The gains of the variable gainamplifiers 132 and 137 are controlled in the opposite directions in arange of from 0 to 3.414 by the outputs Ef and Eb of phase discriminatoras the control unit 49 for controlling rear and front channels bydetecting the phase difference between the signals L and R. The outputFL1 from the matrix circuit 133 is coupled to one input of an adder 179via a l/ 2 attenuator 180 and the output FL3 from the matrix circuit 175is applied to the other input of adder 179. Similarly, the output FRlfrom the matrix circuit 133 is applied to one input of an adder 181through a l/ 2 attenuator 182 whereas the output FR3 from the matrixcircuit 177 is coupled to the other input of adder 181. Likewise, theoutput RL1 of matrix circuit 138 is applied to one input of an adder 183through a l/ \f2 attenuator 184 while the output RL3 of matrix circuit175 is applied to the other input of adder 183. The output RRl of matrixcircuit 138 is coupled to one input of adder 185 through a l V2attenuator 186 and the output RR3 of the matrix circuit 177 is appliedto the other input of adder 185.

It will be clear that the four channel signals FL, FR, RL and RR areexpressed by the following equations.

FIG. 14 shows a block diagram of a modification of the variable matrixcircuit shown in FIG. 13. In FIG. 14, variable gain amplifiers 132 and137 are controlled respectively by the outputs Ef and Eb from acomparator as the first control unit 49 for the front-rear control whichdetects the difference in the levels of the sum signal (L+R) and thedifference signal (L--R), and variable gain amplifiers 176 and 178 arecontrolled respectively by the outputs El and Er from a comparator 190as the second control unit for the left and right control which detectsthe difference in the levels of the left signal L and the right signalR. In this manner, it is possible to obtain the same effect as thevariable matrix circuit shown in FIG. 13 by detecting the difference inthe signal levels.

A conventional 2-channel stereophonic disc is so designed that thereproduced sound image of a low-tone instrument may be localized midwaybetween the left and right loudspeakers. To use such a 2-channelstereophonic disc for 4-channel sound reproduction, the sound images oflow-tone and high-tone instruments may better be localized respectivelyat the front and back of the reproduced sound field, thereby to effect adesirable 4-channel sound reproduction. To this end, the characteristicsof the converter 11 should be such that two composite signals L and Rproduced by the converter will possess a phase relationship which variesin accordance with the frequency of the input signals. In other words,the phase difference between the composite signals L and R shouldapproach 180 as the frequencies of input signals L and R grow higher.Shown in FIG. 15 is an embodiment of a preferable signal converterhaving such phasefrequency characteristics and preferably combined witha variable matrix circuit.

In FIG. 15, a transistor Q has its base connected to an input terminal21, and a capacitor C and a resistor R are connected in series betweenthe collector and emitter of the transistor Q with their junctionconnected to an output terminal 23. A collector resistor R and anemitter resistor R of the same value are provided for the transistor QThe transistor Q the resistors R R and R and the capacitor C constitutea phase shifter of well-known type. Similarly, a transistor Q, has itsbase connected to an input terminal 22, and a capacitor C and a resistorR are connected in series between the collector and emitter of thetransistor Q6 with their junction connected to an output terminal 24. Acollector resistor R and an emitter resistor R of the same value areprovided for the transistor Q Another resistor R is used to connect theemitters of the transistors Q 5 and Q The signal converter of theabove-mentioned construction functions in the following manner. Betweeninput signal L supplied to the input terminal 21 and output signal Lappearing at the output terminal 23 there is observed a phase differencehaving such a frequency dependency as represented by curve I in FIG. 16.If input signal L has a frequency f defined by the capacitor C and theresistor R (e.g. when C 0.01 microfarad and R 22 kiloohms, f /zrrC R 723Hz), the resultant phase difference is 90. Thus the phase differencegoes near --l as the frequency of the input signal L grows higher. Asproved by curve II in FIG. 17, however, the gain of the phase shifterincluding the transistor Q, is constant with respect to signal Lregardless of the changes of frequency of signal L.

Signal L coming out of the emitter of the transistor Q is attenuated byan attenuator constituted by the re sistors R and R and appears at theoutput terminal 24 through a high-pass filter constituted by thecapacitor C and the resistor R Signal L undergoes, at the highpassfilter, such a phase variation as indiated by curve II in FIG. 16 and,at the attenuator and high-pass filter, such a gain variation asdepicted by curve IV in FIG. 17. The phase of signal L is varied by 45at the highpass filter if the frequency of signal L is f Theabovementioned operation of the signal converter is applicable also tothe input signal R.

As mentioned above, the phase difference between the output signals Land R from the signal converter is about when input signal L has afrequency of low frequency band. However, level amplitude lelvel ofoutput signal R can be negligibly small compared with output signal L.If input signal L has a frequency of high frequency band, output signalsL and R are opposite in phase to each other. Accordingly, so long asinput signals L and R are of frequencies of low frequency band, thephase relationship between output signals L and R can never be reversedeven in only one of input signals is supplied to the input terminal, andsignals having frequencies of low frequency band are never localized atthe back of the reproduced sound field. If input signals L and R are ofan equal frequency of low frequency band, that is, if they are to be.localized midway between the left and right loudspeakers, they will belocalized at the front of the reproduced sound field. If only inputsignal L is of a frequency of high frequency band, it will be localizedat the rear-left loudspeaker. And if only signal R is of a frequency ofhigh frequency band, it will be localized at the rearright loudspeaker.

FIG. 11 shows another embodiment of this invention in which a signalconverter is used for the two-channel playback of a matrix four-channelsource. The first and second audio signals L and R reproduced from thematrix four-channel source which may be a matrix fourchannel stereorecord, a matrix four-channel magnetic tape or a matrix four-channel FMstereo signal source are expressed by the following equation;

L =FL AFR +jRL +jARR R= FR AFL jRR jARL Where only the rear-left signalRL presents, L RL and R ARL. Where signal converter 11 shown in FIG. 5or 6 is adjusted to provide two sum signals L L Br and R R BL which areapplied to a conventional Z-channel reproducer 122, the signal converterdescribed above will produce outputs L RL ABRL and R RL BRL.Accordingly, outputs L (lA )RL and R can be produced by a properadjustment of B. This means that it is possible to make infinity theseparation between rear signals. Thus, it will be seen thatincorporation of a signal converter which produces sum signals into acombination of a matrix four-channel source containing rear signals ofan especially large level and a two-channel reproducing system isadvantageous in the reproduction of a matrix fourchannel sound source bya two-channel system.

Where only a front left signal FL presents, L FL and R AFL. When thepotentiometer resistors of the signal converter shown in FIGS. and 6 areadjusted to provide difference signals L L OR and R R aL respectively,the outputs will become L' FL AaFL and R AFL aFL. Thus, it is possibleto obtain outputs L (1A )FL and R 0 by a proper adjustment of a. Thisshows that it is possible to make infinity the separation between frontsignals. Accordingly, when a signal converter 11 as shown in FIG. 5 or 6is used in the circuit shown in FIG. 11 it is possible to control theseparation between rear signals or front signals in accordance with thecontent recorded on a matrix four-channel recording medium. In the caseof a jazz, the level of the rear signal is relatively high whereas inthe case of a classical music the level of the rear signal is low.

What we claim is:

1. A sound reproducing system for reproducing stereophonically relatedfirst and second channel signals from a two-channel signal source bymeans of a fourchannel reproducing system, said system comprising acombination of:

first means connected to receive the first and second channel signalsfor producing first and second composite signals by combining the firstand second channel signals with such amplitude and phase relationshipsthat the first composite signal contains a greater proportion of thefirst channel signal and a smaller proportion of the second channelsignal, the second composite signal contains a greater proportion of thesecond channel signal and a smaller proportion of the first channelsignal, and for producing phase differences between the first and secondcomposite signals with respect to the first and second channel signalsrespectively which vary as a function of frequency of the first andsecond channel signals;

control means connected to receive the first and second compositesignals for producing control outputs the levels of which vary dependingon the phase relationship between the first and second compositesignals; and

second means connected to receive, as first and second input signals,the first and second composite signals or the first and second channelsignals for producing four-channel output signals by combining the firstand second input signals, said second means being operative to producethe four-channel output siganls varying at least mixing ratios of thefirst and second input signals depending on the levels of the controloutputs of said control means.

2. A sound reproducing system according to claim 1 wherein the phasedifferences between the first and second composite signals with respectto the first and second channel signals represent substantially at a lowfrequency and substantially at a high frequency, respectively.

3. A sound reproducing system according to claim 1 wherein the amplitudelevels of the second and first channel signals contained in the firstand second composite signals vary as a function of frequency,respectively.

4. A sound reproducing system according to claim 1 wherein said firstmeans comprises first and second phase shifter means each including atransistor having a collector, and emitter and a base, and a resistorand a capacitor connected in series across the collector and emitter ofthe transistor; and a resistor means connecting the emitter oftransistor in said first phase shifter means with the emitter oftransistor in said second phase shifter means, the bases of transistorsin said first and second phase shifter means being connected to receivethe first and second channel signals, respectively, theresistor-capacitor junctions in said first and second phase shiftermeans being adapted to derive the first and second composite signals,respectively.

5. A sound reproducing system according to claim 1 wherein said secondmeans comprises a first differential amplifier having first and secondoutput terminals deriving a pair of output signals and first and secondinput terminals, the first input terminal being connected to receive thefirst input signal; phase reversing means for reversing the phase of thesecond input signal; means for coupling the output of said phasereversing means to the second input terminal of said first differentialamplifier; a second differential amplifier having first and secondoutput terminals deriving another pair of output signals and first andsecond input terminals connected to receive the first and second inputsignals, respectively; and means for controlling the gains of said firstand second differential amplifiers in opposite relationship inaccordance with the levels of control outputs from said control means.

6. A second reproducing system according to claim 1 wherein said secondmeans comprises means connected to receive the first and second inputsignals A and B for producing a first output signal substantiallyproportional to (1+j)A (lf)B where f represents a first variable matrixcoefficient; means connected to receive the first and second inputsignals A and B for producing second output signal substantiallyproportional to (1+f)B l-f)A; means connected to receive the first andsecond input signals A and B for producing a third output signalsubstantially proportional to (l+b)A (1-b)B where b represents a secondvariable matrix coefficient; means connected to receive the first andsecond input signals A and B for producing a fourth output signalsubstantially proportional to (1+b)B (lb)A; and means for varying thematrix coefficients f and b in opposite relationship depending on thelevels of control outputs from said control means.

7. A sound reproducing system according to claim 1 wherein said controlmeans includes a phase discriminator for detecting the phase differencebetween the first and second composite signals.

8. A sound reproducing system according to claim 1 wherein said controlmeans includes a level comparator for comparing the levels of sum anddifference of the first and second composite signals.

9. A sound reproducing system according to claim 1 further comprisingfurther control means for producing control outputs in response to thefirst and second composite signals; and wherein said second meanscomprises means connected to receive the first and second input signalsA and B for producing a first output signal substantially proportionalto (l+f+ VZA (1f 51 )8, where f and I represent variable matrixcoefficients; means connected to receive the first and second inputsignals A and B for producing a second output signal substantiallyproportional to (1+f+ E)B (lf+ 2 r)A, where r represent a variablematrix coefficient; means connected to receive the first and secondinput signals A and B for producing a third output signal substantiallyproportional to (l+b+ 2 )A (lb+ VEDB, where b represent a variablematrix coefficient; means connected to receive the first and secondinput signals A and B for producing a fourth output signal substantiallyproportional to (l+b+ )B (1-b+ V 2 r)A; and means for varying the matrixcoefficients f and b in opposite relationship depending on the levels ofcontrol outputs from said control means; and means for varying thematrix coefficients l and r in opposite relationship depending on thelevels of control outputs from said further control means.

10. A sound reproducing system according to claim 9 wherein said furthercontrol means includes a phase discriminator for detecting phasedifference between sum and difference of the first and second compositesignals.

11. A sound reproducing system according to claim 9 wherein said furthercontrol means includes a level comparator for comparing the levels ofthe first and second composite signals.

12. A sound reproducing system for reproducing stereophonically relatedfirst and second channel signals from a two-channel signal source bymeans of a fourchannel reproducing system, said system comprising acombination of:

first means connected to receive the first and second channel signalsfor producing first and second composite signals by combining the firstand second channel signals with such amplitude and phase relationshipsthat the first composite signal contains a greater proportion of thefirst channel signal and a smaller proportion of the second channelsignal, the second composite signal contains a greater pro portion ofthe second channel signal and a smaller proportion of the first channelsignal, and for producing phase differences between the first and secondcomposite signals with respect to the first and second channel signalsrespectively vary as a function of frequency of the first and secondchannel signals;

first control means connected to receive the first and second compositesignals for producing control outputs the levels of which vary dependingon the phase relationship between the first and second compositesignals;

second control means connected to receive the first and second compositesignals for producing control outputs the levels of which vary dependingon the phase relationship between sum and difference of the first andsecond composite signals;

second means connected to receive, as first and second input signals,the first and second composite signals or the first and second channelsignals for producing four-channel output signals by combining the firstand second input signals, said second means including means for varyingthe mixing ratios of and polarity relationship between the first andsecond input signals in producing the output signals depending on thelevels of control outputs from said first control means; and means forvarying the mixing ratios of the first and second input signals inproducing the output signals depending on the levels of control outputsfrom said second control means.

13. A sound reproducing system for reproducing stereophonically relatedfirst and second channel signals from a two-channel signal source bymeans of a fourchannel reproducing system, said system comprising acombination of:

first means connected to receive the first and second channel signalsfor producing first and second composite signals by combining the firstand second channel signals with such amplitude and phase relationshipsthat the first composite signal contains a greater proportion of thefirst channel signal and a smaller proportion of the second channelsignal, the second composite signal contains a greater proportionof thesecond channel signal and a smaller proportion of the first channelsignal, and for producing phase differences between the first and secondcomposite signals with respect to the first and second channel signalsrespectively vary as a function of frequency of the first and secondchannel signals;

control means connected to receive the first and second compositesignals for producing control outputs the levels of which vary dependingon the phase relationship between the first and second compositesignals; and

second means connected to receive, as first and second input signals,the first and second composite signals or the first and second channelsignals for producing four-channel output signals by combining the firstand second input signals, said second means including means for varyingthe mixing ratios of and polarity relationship between the first andsecond input signals in producing the output signals depending on thelevels of control outputs from said control means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO.3,889,061 DATED June 10, 1975 INVENTOR(S) Ryosuke Ito and SusumuTakahashi It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Add on first page of patent:

[30] Foreign Application Priority Data December 21, 1971 Japan 103970/71Signed and Scaled this A tres r:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ommissinncr uj'lalenrsand Trademarks

1. A sound reproducing system for reproducing stereophonically relatedfirst and second channel signals from a two-channel signal source bymeans of a four-channel reproducing system, said system comprising acombination of: first means connected to receive the first and secondchannel signals for producing first and second composite signals bycombining the first and second channel signals with such amplitude andphase relationships that the first composite signal contains a greaterproportion of the first channel signal and a smaller proportion of thesecond channel signal, the second composite signal contains a greaterproportion of the second channel signal and a smaller proportion of thefirst channel signal, and for producing phase differences between thefirst and second composite signals with respect to the first and secondchannel signals respectively which vary as a function of frequency ofthe first and second channel signals; control means connected to receivethe first and second composite signals for producing control outputs thelevels of which vary depending on the phase relationship between thefirst and second composite signals; and second means connected toreceive, as first and second input signals, the first and secondcomposite signals or the first and second channel signals for producingfour-channel output signals by combining the first and second inputsignals, said second means being operative to produce the four-channeloutput siganls varying at least mixing ratios of the first and secondinput signals depending on the levels of the control outputs of saidcontrol means.
 2. A sound reproducing system according to claim 1wherein the phase differences between the first and second compositesignals with respect to the first and second channel signals representsubstantially 90* at a low frequency and substantially 180* at a highfrequency, respectively.
 3. A sound reproducing system according toclaim 1 wherein the amplitude levels of the second and first channelsignals contained in the first and second composite signals vary as afunction of frequency, respectively.
 4. A sound reproducing systemaccording to claim 1 wherein said first means comprises first and secondphase shifter means each including a transistor having a collector, andemitter and a base, and a resistor and a capacitor connected in seriesacross the collector and emitter of the transistor; and a resistor meansconnecting the emitter of transistor in said first phase shifter meanswith the emitter of transistor in said second phase shifter means, thebases of transistors in said first and second phase shifter means beingconnected to receive the first and second channel signals, respectively,the resistor-capacitor junctions in said first and second phase shiftermeans being adapted to derive the first and second composite signals,respectively.
 5. A sound reproducing system according to claim 1 whereInsaid second means comprises a first differential amplifier having firstand second output terminals deriving a pair of output signals and firstand second input terminals, the first input terminal being connected toreceive the first input signal; phase reversing means for reversing thephase of the second input signal; means for coupling the output of saidphase reversing means to the second input terminal of said firstdifferential amplifier; a second differential amplifier having first andsecond output terminals deriving another pair of output signals andfirst and second input terminals connected to receive the first andsecond input signals, respectively; and means for controlling the gainsof said first and second differential amplifiers in oppositerelationship in accordance with the levels of control outputs from saidcontrol means.
 6. A second reproducing system according to claim 1wherein said second means comprises means connected to receive the firstand second input signals A and B for producing a first output signalsubstantially proportional to (1+f)A + (1-f)B where f represents a firstvariable matrix coefficient; means connected to receive the first andsecond input signals A and B for producing second output signalsubstantially proportional to (1+f)B + (1-f)A; means connected toreceive the first and second input signals A and B for producing a thirdoutput signal substantially proportional to (1+b)A - (1-b)B where brepresents a second variable matrix coefficient; means connected toreceive the first and second input signals A and B for producing afourth output signal substantially proportional to (1+b)B - (1-b)A; andmeans for varying the matrix coefficients f and b in oppositerelationship depending on the levels of control outputs from saidcontrol means.
 7. A sound reproducing system according to claim 1wherein said control means includes a phase discriminator for detectingthe phase difference between the first and second composite signals. 8.A sound reproducing system according to claim 1 wherein said controlmeans includes a level comparator for comparing the levels of sum anddifference of the first and second composite signals.
 9. A soundreproducing system according to claim 1 further comprising furthercontrol means for producing control outputs in response to the first andsecond composite signals; and wherein said second means comprises meansconnected to receive the first and second input signals A and B forproducing a first output signal substantially proportional to (1+f+Square Root 2)A + (1-f + Square Root 2l)B, where f and l representvariable matrix coefficients; means connected to receive the first andsecond input signals A and B for producing a second output signalsubstantially proportional to (1+f+ Square Root 2)B + (1-f+ Square Root2r)A, where r represent a variable matrix coefficient; means connectedto receive the first and second input signals A and B for producing athird output signal substantially proportional to (1+b+ Square Root2)A - (1-b+ Square Root 2l)B, where b represent a variable matrixcoefficient; means connected to receive the first and second inputsignals A and B for producing a fourth output signal substantiallyproportional to (1+b+ Square Root 2)B - (1-b+ Square Root 2r)A; andmeans for varying the matrix coefficients f and b in oppositerelationship depending on the levels of control outputs from saidcontrol means; and means for varying the matrix coefficients l and r inopposite relationship depending on the levels of control outputs fromsaid further control means.
 10. A sound reproducing system according toclaim 9 wherein said further control means includes a phasediscriminator for detecting pHase difference between sum and differenceof the first and second composite signals.
 11. A sound reproducingsystem according to claim 9 wherein said further control means includesa level comparator for comparing the levels of the first and secondcomposite signals.
 12. A sound reproducing system for reproducingstereophonically related first and second channel signals from atwo-channel signal source by means of a four-channel reproducing system,said system comprising a combination of: first means connected toreceive the first and second channel signals for producing first andsecond composite signals by combining the first and second channelsignals with such amplitude and phase relationships that the firstcomposite signal contains a greater proportion of the first channelsignal and a smaller proportion of the second channel signal, the secondcomposite signal contains a greater proportion of the second channelsignal and a smaller proportion of the first channel signal, and forproducing phase differences between the first and second compositesignals with respect to the first and second channel signalsrespectively vary as a function of frequency of the first and secondchannel signals; first control means connected to receive the first andsecond composite signals for producing control outputs the levels ofwhich vary depending on the phase relationship between the first andsecond composite signals; second control means connected to receive thefirst and second composite signals for producing control outputs thelevels of which vary depending on the phase relationship between sum anddifference of the first and second composite signals; second meansconnected to receive, as first and second input signals, the first andsecond composite signals or the first and second channel signals forproducing four-channel output signals by combining the first and secondinput signals, said second means including means for varying the mixingratios of and polarity relationship between the first and second inputsignals in producing the output signals depending on the levels ofcontrol outputs from said first control means; and means for varying themixing ratios of the first and second input signals in producing theoutput signals depending on the levels of control outputs from saidsecond control means.
 13. A sound reproducing system for reproducingstereophonically related first and second channel signals from atwo-channel signal source by means of a four-channel reproducing system,said system comprising a combination of: first means connected toreceive the first and second channel signals for producing first andsecond composite signals by combining the first and second channelsignals with such amplitude and phase relationships that the firstcomposite signal contains a greater proportion of the first channelsignal and a smaller proportion of the second channel signal, the secondcomposite signal contains a greater proportion of the second channelsignal and a smaller proportion of the first channel signal, and forproducing phase differences between the first and second compositesignals with respect to the first and second channel signalsrespectively vary as a function of frequency of the first and secondchannel signals; control means connected to receive the first and secondcomposite signals for producing control outputs the levels of which varydepending on the phase relationship between the first and secondcomposite signals; and second means connected to receive, as first andsecond input signals, the first and second composite signals or thefirst and second channel signals for producing four-channel outputsignals by combining the first and second input signals, said secondmeans including means for varying the mixing ratios of and polarityrelationship between the first and second input signals in producing theoutput signals depending on the levels of control outputs from saidcontrol means.